FIG. 1a illustrates an exemplary cell site 2 for use within a communications network such as a cellular communications network. The cell site 2 includes a radio server 4 and a cell tower 12. The radio server 4 includes a modem 6, modem software 8, a network interface, an operation and maintenance processor and software, and soft handoff switch software. The cell tower 12 includes one or more antennae 14 mounted to the top of the cell tower for transmitting and receiving wireless communication signals.
The cell site 2 also includes one or more transceivers or radio heads (RHs) 16. Each RH 16 includes a power amplifier, digital-to-analog (D/A) converters, analog-to-digital (A/D) converters, radio frequency (RF) upconverter (UC), RF downconverter (DC), and digital signal processing circuitry 10 for communicating over multiple network protocols. The RHs 16 may be located within the same box as the radio server 4 (e.g. on one or more cards in one or more slots in a rack-mounted configuration). When the radio server 4 and the RHs 16 are located within the same box, they may be referred to as “Node B” or as a basestation.
Alternatively, the RHs 16 may be located in a separate housing from the radio server 4 but connected to an antenna on the top of the cell tower through a lossy cable, or mounted at the top of the cell tower 12 near the antennae 14, which reduces the connection loss between the RHs and the antenna. When the RHs 16 are not located in the same box as the radio server 4, they may be referred to as “remote” radio heads (RRHs). When the RRHs are located at the top of the cell tower 12, they may be referred to as tower-mounted RRHs.
FIG. 1b illustrates a cluster of cell sites 2 in which there is a single radio server 4 connected by fiber optic lines 18 in a daisy chain or parallel configuration to multiple remote radio heads (RRHs) 20, each RRH located at a different cell site. The RRHs 20 may be located at the base of the cell tower 12 at each cell site 2 or alternatively at the top of the cell tower in a tower-mounted configuration.
As mentioned above, the RHs and RRHs of FIGS. 1a and 1b contain power amplifiers (PAs). The output power levels of the PAs may change over time as a function of the number of users. In general, as the number of users increases or the amount of traffic increases (e.g. if multiple users are downloading data), the output power levels increase. In addition, because each user is under power control, as the user gets closer to the cell site 2 or farther away from the cell site, the output power level transmitted to that user decreases or increases accordingly.
FIG. 2 illustrates an exemplary power amplifier characteristic curve of input power (x-axis) versus output power (y-axis). As FIG. 2 illustrates, at higher input power levels the curve compresses at 60 and becomes non-linear, so that the actual amount of output power is less than what is expected under ideal conditions. Besides this amplitude distortion, the power amplifier exhibits non-linear dynamics characteristics otherwise known as “memory effect distortion” and phase distortion. These four PA characteristics comprise the major PA distortion effects and collectively cause output power “signal distortion.”
In historical second generation (2G) cellular communication services, such as GSM, GPRS, or EDGE which uses GMSK or in the case of EDGE 3 pi/8 MSK modulations, class C PAs were used to amplify a modulated carrier with a relatively high efficiency approaching 50% power added efficiency (PAE). No linearization of the output power versus input power curve was required, because the output signal was provided at a constant amplitude or very small peak-average-ratio (PAR) in the case of an EDGE signal. With current third generation (3G) cellular communication services, Gaussian-like signals are generated with large PAR, and class AB PAs or the more efficient but highly non-linear Doherty PAs are required.
Current-generation PAs are generally expensive and show low DC to RF conversion efficiency and therefore account for the main part of the heat generated by transmitter systems. PAs not only generate non-linear distortions but also possess memory effects that contribute to the nonlinear behavior significantly once the input excitation has wide instantaneous bandwidth.
The transmit signal is a modulated signal and thus consists of various frequency contents, expressed as follows:
                              x          ⁡                      (            t            )                          =                              ∑            i                    ⁢                                    x              i                        ⁡                          (                              t                ,                                  f                  i                                            )                                                          (        0.1        )            When this signal is passed through the transmitter chain comprised of a digital to analog converter (DAC), radio frequency (RF) electronics and the PA, the signal undergoes different distortions: (1) Static Non-Linear Distortion (due to frequency translation from IF to RF and more so in amplifier stage); (2) Non-linear Dynamic Distortion known as PA Memory Effect; (3) Amplitude distortions (due to non-ideal filtering); (4) Phase distortions (due to non-ideal filtering); and (5) Time Delay distortions (due to group delay variations in filtering).
In addition, the PA characteristics change with temperature. As the transmit signal rapidly changes levels, the thermal effects of the PA change, which cause the PA characteristics to change. Since the signal source is typically dynamic and the amplitude can vary 5-10 dB within a very short period (e.g. for HSDPA, High Speed Downlink Packet Access), the PA gain and phase characteristics can change fairly rapidly.
Without linearization, the efficiency of the class AB PAs in 3G cellular communication services drops significantly and would be estimated to be around 4%. Thus, there is a need to improve the efficiency of the PAs in 3G cellular communication services.
Using analog techniques, efficiency can be improved to about 8%. Conventional digital techniques can raise this efficiency to about 20-25% using Class AB power amplifiers. However, there is still a need to improve PA efficiency even more while maintaining good Channel Power Leakage (CPL). When applying conventional DPD techniques with a high efficiency PA (such as Doherty PA), the CPL of the PA signal output is degraded (and could fail the Spectral Emission Mask (SEM) requirement) especially for transitioning signals, where the signal can be transitioned from low power to high power in a rapid fashion. Therefore, the conventional approach is not practical for high efficiency PA's.
“Pre-distortion” is a known technique for applying a pre-distorted PA input signal to a PA to cancel out or compensate for the inherent distortion of the PA and improve the linearization and therefore the efficiency of the PA. However, previous digital implementations utilized digital signal processing (DSP) and software, which can be too slow for current PAs that can experience rapid changes to power levels. In addition, any previous digital implementations were not optimized to work with highly non-linear PAs such as a Doherty pair nor would fit on a single chip.